This invention relates to an electroluminescent panel for use in displaying alphanumeric symbols, a static picture, a motion picture, and the like in an input/output device of a computer, and so on.
A conventional electroluminescent panel of the type described comprises a transparent substrate, a transparent or first electrode on the substrate, a back or second electrode opposite to the transparent electrode, and an intermediate layer between the first and the second electrodes. The intermediate layer comprises a dielectric layer and an electroluminescent layer of, for example, zinc sulfide (ZnS) which contains manganese (Mn).
With this structure, the electroluminescent panel is luminous in a yellowish orange color when an electric voltage is supplied between the first and the second electrodes. This is because electrons are excited to a conduction band by the electric field and are accelerated to activate luminescent centers of the manganese from a ground state. Such activated luminescent centers emit in the yellowish orange light when they return to the ground state.
The dielectric layer included in the intermediate layer serves to give a favorable value to the electric field developed in the electroluminescent layer. In addition, the dielectric layer is useful in raising the breakdown voltage of the electroluminescent panel.
Under the circumstances, it is preferable that the dielectric layer has a high dielectric strength, a high relative dielectric constant, a low dielectric loss, and a high moisture-proof capability. In addition, the dielectric layer should be in intimate contact with an adjacent layer.
Heretofore, yttrium oxide, aluminum oxide, silicon nitride, silicon oxynitride, tantalum pentoxide, lead titanate, barium titanate, and the like are used as materials of the dielectric layer. However, layers of the materials enumerated above have merits and demerits. More particularly, the yttrium oxide layer has a high relative electric constant between 10 and 15 and may therefore be convenient for, experimental use. However, the yttrium oxide layer has poor moisture-proof capability.
The aluminum oxide layer, the silicon nitride layer, and the silicon oxynitride layer have low relative dielectric constants, although they have enough high moisture-proof capabilities. Therefore, a high electric voltage should be given to the electroluminescent panel comprising either one of such layers.
The dielectric layer of either lead titanate or barium titanate is not only liable to be variable in composition but also low in dielectric strength because each of lead titanate and barium titanate is crystalline.
The tantalum pentoxide layer is favorably dense and has a high relative dielectric constant between 20 and 30, when deposited by sputtering. However, it has weak adhesion to the electroluminescent layer and the back electrode and is liable to peel off during manufacture of the electroluminescent panel and/or during operation thereof.
In order to avoid the above-mentioned defects, attempts have been directed to a composite intermediate layer comprising as the dielectric layer a plurality of dielectric films which are different in composition from one another. However, an electroluminescent panel of this type is disadvantageous because of insufficient adhesion between two adjacent layers and serious dielectric loss. In addition, deposition of such a composite intermediate layer is very troublesome.